When cooking frozen foods and the like in a microwave oven, it is desired to heat the foodstuff not only from penetration of microwave energy but also by convection and conduction from the receptacle on which the food article is positioned. An early concept, suggested by Turpin U.S. Pat. No. 4,190,757 was to use a composite material which contained a lossy substance which permitted a portion of the microwave energy to pass through the material for heating the foodstuffs by microwave absorption while also being heated by the microwave energy so that the support material itself could additionally heat the food. This concept was further refined in Seiferth U.S. Pat. No. 4,641,005 which produced Turpin's lossy material by vacuum depositing a very thin film of microwave interactive metal having a surface sensitivity expressed in ohms per inch onto a plastic film which in turn was bonded to an article support surface. When it was found that the support surface for Seiferth's tape could comprise paperboard, typically 16 point paperboard with a rigidity of standard posterboard stock, widescale commercial use of the susceptor sheet stock to form plates, boats and platforms for reconstitution of frozen food in microwave ovens resulted. This invention uses conventional susceptor sheet stock which is defined as a generally continuous, microwave interactive material formed by vacuum depositing a thin layer of aluminum or similar microwave interactive metal onto a smooth plastic support film as taught by Seiferth which in turn is adhered to a flat of generally rigid paperboard.
It has been found that certain frozen foods, such as pizza pies, when reconstituted in a microwave oven, required specially formed susceptor sheet stock receptacles which could heat the crust. Examples of susceptor sheet stock sleeves supplied within the food package are disclosed in Brown U.S. Pat. No. 4,780,587 and Swiontek U.S. Pat. No. 4,794,005. Both Brown '587 and Swiontek have certain package characteristics similar to that of the invention disclosed herein, but Brown and Swiontek receptacles are preformed. Brown U.S. Pat. No. 4,626,641 illustratrates a consumer formed container having portions of microwave susceptor sheet stock. Jaegar U.S. Pat. No. 4,891,482, owned by the assignee of this invention, particularly advanced the art by developing a pop-up sleeve formed of susceptor sheet stock which fit within the frozen food package in a folded position. When the package was opened, the consumer simply removed the sleeve, popped it to its open position and inserted the sleeve with the pizza therein into the microwave oven for cooking. While the sleeve functioned acceptably to heat the pizza and other food crusted materials, there were some disadvantages to the sleeve from a manufacturing and packaging point of view. With respect to the pop-up sleeve embodiment in Jaeger, the edges of the sleeve had to be glued to form a collapsible container increasing material and assembly costs and when the sleeve was collapsed into its initial position, the sleeve took up more space within the carton than otherwise desired. Also, after the pizza was heated within the sleeve, it became difficult to remove the pizza from the sleeve necessitating, in some instances, the consumer cutting the sleeve. It is to be recognized that in order for the microwave susceptor sheet stock to efficiently perform the desired heating, close spaces between the foodstuff and container are required and removal of the heated food can present difficulty to the consumer who attempts to remove a hot item with his or her bare hands. These problems were recognized somewhat in Jaeger and overcome to some extent by an alternative embodiment which used tabs to form a box from a flat sheet of susceptor stock thus obviating the glue step required in the sleeve. However, the box had closed ends when assembled thus preventing air flow and convection heating therethrough which can be achieved with the sleeve. Also, the tabs were unwieldy and it was somewhat difficult to form the box from its flat condition. In addition, a separate spacer sheet had to be provided. Finally, the tab and end wall configuration materially protruded from the body portion of the box in the flat condition. Because the box is cut from a larger sheet of susceptor sheet stock or paperboard, this increases the material usage over that otherwise possible and also adversely dictates the final carton dimensions.